The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The measurement of fluorescence lifetime and its application for biology and medicine have been studied intensively. Measuring fluorescence lifetimes of fluorophores provides a sensitive probe of the microenvironments of the fluorophore. Its sensitivity is not affected by the concentration of the fluorophore or by the excitation light intensity. This is particularly significant in biological and clinical applications where dye concentration cannot be precisely controlled and light fluence is highly non-uniform. Optical imaging techniques have been developed to map the distribution of fluorescence lifetime in a wide field of view. These techniques were applied for functional imaging of cellular metabolism, oxygen sensing, mapping the concentration and dynamics of ions such as Ca, and for probing molecular associations by sensing intra-molecular distances. In all these applications the lifetime is evaluated by measuring the decay of the fluorescent signal, or by transient absorption. One of the major difficulties in applying these methods for clinical imaging is the loss of spatial information due to strong light scattering.
According to the principles of the present teachings, a method for local measurement of a dye's excited state lifetime is provided. This method does not require collecting the emitted light but rather relies on photoacoustic probing. This method is demonstrated for oxygen level measurement using oxygen sensitive dye and also for pH imaging using ratiometric photoacoustic chemical sensing.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.